Laser beam optical focusing system of two symmetrical diffractive optical elements

ABSTRACT

A laser beam optical focusing system consists of two symmetrical diffractive optical elements. The optical system has a spot size of less than one micron and a Strehl ratio of over 0.9 with a f/number of one and a total object/image field of either 0.5 or 1 micron.

BACKGROUND OF THE INVENTION

This invention relates to a laser beam optical focusing system and, moreparticularly, to an optical focusing system with two symmetricaldiffractive optical elements.

Laser beams have wide-ranging applications such as heat annealing orvaporization, optical memory recording and playback, optical scanningand xerographic and printing use. For these diverse applications, alaser beam should be a focused beam. Various optical systems have beenproposed over the years to provide focusing means for a laser beam.

Optical focusing systems in modern day apparatus are becoming moreaccurate on the one hand but more complicated and expensive on the otherhand.

A compact design for the focusing optics of any optical system is alwaysdesirable to make the entire optical system itself as compact aspossible and to enable extension of the same design into manyarchitectures.

It would be desirable to improve the efficiency, shorten optical pathlengths and use as few optical elements as possible to decreasehardware, assembly and alignment costs of these types of opticalfocusing systems.

A semiconductor laser or laser diode emits a diverging light beam. Theprior art uses optical elements, typically lenses but sometimes mirrors,to shape and focus the angular diverging beam emitted from asemiconductor laser to a focused spot size.

A single field point in a prior art focusing system with refractivelenses can easily be optimized given enough degrees of freedom in thelenses. However, with a large field of points and a small spot size, thedesign of an optical focusing refractive lens system is difficult.

Prior art focusing systems typically involve several refractive lenselements configured in an array of lenses. All the lens elements wouldhave to be virtually identical. Fabrication of identical lens elementspresents a problem when tolerances have to be on the order of 1 micronor better for refractive elements. Fabrication of lenses by moldinginvolves heat. Heat can cause differences in thickness and refractiveindex from fabricated lens to fabricated lens, even if the lens are partof the same batch under identical manufacturing circumstances. Also,axial alignment of lens within a focusing system is close to impossible.

The propagation of a light beam can be changed by three basic means:reflection by a mirror, refraction by a lens and diffraction by agrating. Optical systems traditionally rely on reflection and refractionto achieve the desired optical transformation. Optical design, based onmirror and lens elements, is a well-established and refined process.Until recently, the problems with diffraction and fabricating highefficiency diffractive elements have made diffractive elementsunfeasible components of optical systems.

The diffractive process does not simply redirect a light beam.Diffraction, unlike refraction and reflection, splits a light beam intomany beams--each of which is redirected at a different angle or order.The percentage of the incident light redirected by the desired angleinto some given diffraction order is referred to as the diffractionefficiency for that order. The diffraction efficiency of a diffractiveelement is determined by the element's surface profile. If the lightthat is not redirected by the desired angle is substantial, the resultwill be an intolerable amount of scatter in the image or output plane ofthe optical system.

Theoretically, on-axis diffractive phase elements consisting of agrating having a given period can achieve 100 percent diffractionefficiency. To achieve this efficiency, however, a continuous phaseprofile within any given period is necessary. The theoreticaldiffraction efficiency of this surface profile is also relativelysensitive to a change in wavelength. By contrast, refractive elementsare relatively wavelength insensitive. The technology for producing highquality, high efficiency, continuous phase profiles of the diffractiongrating does not presently exist.

A compromise that results in a relatively high diffraction efficiencyand ease of fabrication is a multi-level phase grating. The larger thenumber of discrete phase levels, the better the approximation of thecontinuous phase function. These multi-level phase profiles can befabricated using standard semiconductor integrated circuit fabricationtechniques.

As disclosed in Binary Optics Technology: The Theory and Design ofMulti-level Diffractive Optical Elements by G. J. Swanson of the LincolnLaboratory at the Massachusetts Institute of Technology, (TechnicalReport 854, Aug. 14, 1989) herewithin incorporated by reference and theresulting U.S. Pat. No. 4,895,790 also herewithin incorporated byreference, a fabrication process starts with a mathematical phasedescription of a diffractive phase profile and results in a fabricatedmulti-level diffractive surface. The first step is to take themathematical phase expression and generate from it a set of masks thatcontain the phase profile information. The second step is to transferthe phase profile information from the masks into the surface of theelement specified by the lens design.

The first step involved in fabricating the multi-level element is tomathematically describe the ideal diffractive phase profile that is tobe approximated in a multi-level fashion. The next step in thefabrication process is to create a set of lithographic masks which areproduced by standard pattern generators used in the integrated circuitindustry.

A substrate of the desired material, such as Ge, ZnSe, Si, and SiO₂, iscoated with a thin layer of photoresist. A first lithographic mask isthen placed in intimate contact with the substrate and illuminated fromabove with an ultraviolet exposure lamp. Alternately, patterngenerators, either optical or electron beam, can expose the thin layerof photoresist. The photoresist is developed, washing away the exposedresist and leaving the binary grating pattern in the remainingphotoresist. This photoresist will act as an etch stop.

The most reliable and accurate way to etch many optical materials is touse reactive ion etching. The process of reactive ion etchinganisotropically etches material at very repeatable rates. The desiredetch depth can be obtained very accurately. The anisotropic nature ofthe process assures a vertical etch, resulting in a true binary surfacerelief profile. Once the substrate has been reactively ion etched to thedesired depth, the remaining photoresist is stripped away, leaving abinary surface relief phase grating.

The process may be repeated using a second lithographic mask having halfthe period of the first mask. The binary phase element is recoated withphotoresist and exposed using the second lithographic mask which hashalf the period of the first mask. After developing and washing away theexposed photoresist, the substrate is reactively ion etched to a depthhalf that of the first etch. Removal of the remaining photoresistresults in a 4 level approximation to the desired profile. The processmay be repeated a third and fourth time with lithographic masks havingperiods of one-quarter and one-eighth that of the first mask, andetching the substrates to depths of one-quarter and one-eighth that ofthe first etch. The successive etches result in elements having 8 and 16phase levels. More masks than four might be used, however, fabricationerrors tend to predominate as more masks are used.

This process is repeated to produce a multilevel surface relief phasegrating structure in the substrate. The result is a discrete,computer-generated structure approximating the original idealizeddiffractive surface. For each additional mask used in the fabricationprocess, the number of discrete phase levels is doubled, hence the name"binary" optical element or, more precisely, a binary diffractiveoptical element.

After only four processing iterations, a 16 phase level approximation tothe continuous case can be obtained. The process can be carried out inparallel, producing many elements simultaneously, in a cost-effectivemanner.

A 16 phase level structure achieves 99 percent diffraction efficiency.The residual 1 percent of the light is diffracted into higher orders andmanifests itself as scatter. In many optical systems, this is atolerable amount of scatter. The fabrication of the 16 phase levelstructure is relatively efficient due to the fact that only fourprocessing iterations are required to produce the element.

After the first etching step, the second and subsequent lithographicmasks have to be accurately aligned to the existing pattern on thesubstrate. Alignment is accomplished using another tool standard to theintegrated circuit industry, a mask aligner.

As noted, the photoresist on the substrate can be exposed with anelectron-beam pattern generator. The e-beam direct-write processeliminates masks and their corresponding alignment and exposureproblems. Binary optics have also been reproduced using epoxy casting,solgel casting, embossing, injection molding and holographicreproduction.

Binary optical elements have a number of advantages over conventionaloptics. Because they are computer-generated, these elements can performmore generalized wavefront shaping than conventional lenses or mirrors.Elements need only be mathematically defined: no reference surface isnecessary. Therefore, diffractive optical elements can be madewavelength-sensitive for special laser systems.

The diffractive optical elements are generally thinner, lighter and cancorrect for many types of aberrations and distortions. It is possible toapproximate a continuous phase profile with a stepwise profile ofdiscrete phase levels.

It is an object of this invention to provide an optical focusing systemwith diffractive optical elements.

It is another object of this invention to provide an optical focusingsystem which is compact and contains inexpensive, easy to manufactureand easy to assemble optical elements.

SUMMARY OF THE INVENTION

In accordance with the present invention, a laser beam optical focusingsystem consists of two symmetrical diffractive optical elements. Theoptical system has a spot size of less than one micron and a Strehlratio of over 0.9 with a f/number of one and a total object/image fieldof either 0.5 or 1 micron.

Other objects and attainments together with a fuller understanding ofthe invention will become apparent and appreciated by referring to thefollowing description and claims taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the optical focusing system withtwo symmetrical diffractive optical elements formed according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference is now made to FIG. 1, wherein there is illustrated an opticalfocusing system 10 as an embodiment of the present invention. Theoptical focusing system 10 has a laser source 12 which emits a diverginglight beam 14 of a single wavelength. The diverging light beam 14 isincident upon a first diffractive optical element 16 which diffracts anddirects the resulting beam 18 past a stop 19 onto a second diffractiveoptical element 20. The second diffractive optical element 20 diffractsthe diffracted beam 18 from the first diffractive optical element anddirects the resulting diffracted beam 22 to a spot 24 in the image plane26.

The two diffractive optical elements 16 and 20 of the optical focusingsystem 10 receive a diverging beam 14 from the laser 12 and focuses thebeam 22 to a spot 24. The optical focusing system 10 is flat field.

The laser source 12 is separated by a distance 28 from the front orfirst or incident surface 30 of the first diffractive optical element16. The first diffractive optical element 16 has a substrate thickness32. The back or second or output surface 34 of the first diffractiveoptical element 16 is separated by a distance 36 from the stop 19.

The stop 19 is separated by a distance 38 from the front or first orincident surface 40 of the second diffractive optical element 20. Thesecond diffractive optical element 20 has a substrate thickness 42. Theback or second or output surface 44 of the second diffractive opticalelement 20 is separated by a distance 46 from the spot 24 on the imageplane 26.

The laser source 12 is separated by a total length distance 47 from thespot 24 on the image plane 26.

The object field 48 and image field 50 are 1 to 1 magnification ratio.In the illustrative examples, the size of the object field 48 at thelaser source 12 and the size of the image field 50 at the spot 24 at theimage plane 26 can be either 0.5 mm or 1.0 mm.

The accuracy of an optical focusing system is traditionally measured bythe spot size and the Strehl ratio.

The optical focusing system 10 of the present invention is designed tofocus a laser beam to a Full Width Half Maximum (FWHM) spot size under 1micron. The spot 24 size for a perfect diffraction limited opticalsystem follows the following formulas:

    Spot size=δλf/#                               Equation 1

    Spot size=δλ/(2 N.A.)                         Equation 2

These two equations determine the smallest spot size obtainable for theoptical focusing system 10. The diffraction limit coefficient, δ, istypically 1 for the FWHM spot size. The wavelength in these illustrativeexamples, λ, is 780 nm.. The f-number, f/#, is the ratio of the focallength over diameter of the exit pupil in an infinite conjugate lens.The numerical aperture, N.A., refers to the cone angle of the light in afinite conjugate system. The N.A. is calculated by:

    N.A.=n sin θ                                         Equation 3

where θ is the half angle 52 of the imaging cone 22 of light and n isthe index of refraction of the diffractive optical elements 16 and 20.

For a perfect optical system, the smaller the f/number, the smaller thespot size. The depth of focus for optical systems is proportional to thesquare of the optical spot size. The criteria for this design is a spotsize 24 of 0.78 microns with a Strehl ratio of 0.9.

The Strehl ratio is a measure of the total aberrations in an imagingsystem, which approaches unity as the value of the total aberrationsapproach zero. Only an unattainable complete absence of aberrationswould result in a Strehl Ratio equal to 1. The Strehl ratio is the ratioof the peak intensity of the optical image in an optical system to thepeak intensity of the optical image a for perfect optical system. AStrehl ratio of 0.8 is generally accepted as a good optical system. AStrehl ratio of over 0.9 quantitatively means the total aberrationsintroduced by the two diffractive optical elements 16 and 20 in theoptical focusing system 10 should be negligible.

An f/1 optical system has a very small depth of focus, on the order ofless than 1 micron. One of the benefits of using binary diffractiveoptical elements over a lens system is that diffractive optical elementsallows the optical focusing system to keep a Strehl ratio over 0.9 witha total object/image field of 0.5 mm or 1 mm.

The two diffractive optical elements 16 and 20 are symmetrical. Adiffractive optical element has a diffractive phase profile on thesurface of the substrate. Each diffractive optical element in theseillustrative examples has a diffractive phase profile on each side ofthe diffractive optical element.

The diffractive phase profile of the incident side 30 of the firstdiffractive optical element 16 corresponds to the diffractive phaseprofile of the output side 44 of the second diffractive optical element20. The diffractive phase profile of the output side 34 of the firstdiffractive optical element 16 corresponds to the diffractive phaseprofile of the incident side 40 of the second diffractive opticalelement 20.

The diffractive optical elements 16 and 20 have circular or rotationalsymmetry.

Since the first and second diffractive optical elements are circularlysymmetrical, the diffractive phase profile, Φ, at the radial position,ρ, can therefore be expressed as

    Φ(ρ)=c.sub.1 ρ.sup.2 +c.sub.2 ρ.sup.4 +c.sub.3 ρ.sup.6 +c.sub.4 ρ.sup.8 +c.sub.5 ρ.sup.10 +c.sub.6 ρ.sup.12 +c.sub.7 ρ.sup.14 +c.sub.8 ρ.sup.16 +c.sub.9 ρ.sup.18 +c.sub.10 ρ.sup.20                                              Equation 4

This diffractive phase profile is fabricated by masks in the photoresiston the surface of the substrate to form the diffractive optical element.

Commercially available CODE V optical design software can be used tooptimize the c_(x) polynomial coefficients for the diffractive phaseprofile Φ. The CODE V will also optimize the curvatures of the surfaces,the thickness and the spacings of the diffractive optical element.

The following are illustrative examples of the optical focusing system10 of the present invention.

EXAMPLE 1:

The wavelength of the emitted light beam 14 from the laser source 12 is780 nm. The object field 48 is 0.5 mm. The distance 28 from the lasersource 12 to the incident surface 30 of the first diffractive opticalelement 16 is 1.148775 mm. The c_(x) polynomial coefficients for thediffractive phase profile of the incident surface 30 of the firstdiffractive optical element are

    ______________________________________    C1:   -1.6880E-01                     C2:     3.5689E-01                                     C3:   -2.6389E-01    C4:   1.5943E-01 C5:     -1.8734E-03                                     C6:   -2.6720E-02    C7:   -1.5680E-01                     C8:     -1.8935E-02                                     C9:   6.9715E-01    C10:  -6.2472E-01.    ______________________________________

The substrate of the first diffractive optical element 16 is BK7 Shottglass which is 0.5 mm thick. The c_(x) polynomial coefficients for thediffractive phase profile of the output surface 34 of the firstdiffractive optical element are

    ______________________________________    C1:   -1.9483E-02                     C2:     -1.4060E-01                                     C3:   -2.1030E-02    C4:   6.0059E-02 C5:     2.9186E-02                                     C6:   -1.2125E-02    C7:   -4.4647E-01                     C8:     -1.0259E-03                                     C9:   8.7927E-02    C10:  1.6600E+00    ______________________________________

The output surface 34 of the first diffractive optical element 16 isseparated from the stop 19 by a distance 36 of 0.475527 mm. The stop 19is separated from the incident surface 40 of the second diffractiveoptical element 20 by a distance 38 of 0.668358 mm. The c_(x) polynomialcoefficients for the incident or first side 40 of the second diffractiveoptical element are

    ______________________________________    C1:   1.9483E-02 C2:     1.4060E-01                                     C3:   2.1030E-02    C4:   -6.0059E-02                     C5:     -2.9186E-02                                     C6:   1.2125E-02    C7:   4.4647E-01 C8:     1.0259E-03                                     C9:   -8.7927E-02    C10:  -1.6600E+00    ______________________________________

The substrate of the second diffractive optical element 20 is BK7 Shottglass which is 0.5 mm thick. The c_(x) polynomial coefficients for thediffractive phase profile of the output surface 44 of the seconddiffractive optical element are

    ______________________________________    C1:   1.6880E-01 C2:     -3.5689E-01                                     C3:   2.6389E-01    C4:   -1.5943E-01                     C5:     1.8734E-03                                     C6:   2.6720E-02    C7:   1.5680E-01 C8:     1.8935E-02                                     C9:   -6.9715E-01    C10:  6.2472E-01    ______________________________________

The output surface 44 of the second diffractive optical element 20 isseparated from the spot 24 on the image plane 26 by a distance 46 of1.148775 mm. The image field 50 is 0.5 mm. The magnification ratio is 1to 1.

The spot size 24 is 0.78 microns with a Strehl ratio of 0.9.

The total length distance 47 of the optical focusing system 10 from thelaser source 12 to the spot 24 on the image plane 26 is 4.441435 mm.

As noted, the two diffractive optical elements 16 and 20 aresymmetrical. The diffractive phase profile of the incident side 30 ofthe first diffractive optical element 16 corresponds to the diffractivephase profile of the output side 44 of the second diffractive opticalelement 20. The diffractive phase profile of the output side 34 of thefirst diffractive optical element 16 corresponds to the diffractivephase profile of the incident side 40 of the second diffractive opticalelement 20.

EXAMPLE 2:

The wavelength of the emitted light beam 14 from the laser source 12 is780 nm. The object field 48 is 0.5 mm. The distance 28 from the lasersource 12 to the incident surface 30 of the first diffractive opticalelement 16 is 1.152816 mm. The c_(x) polynomial coefficients for thediffractive phase profile of the incident surface 30 of the firstdiffractive optical element are

    ______________________________________    C1:   -1.6929E-01                     C2:     3.5436E-01                                     C3:   -2.6430E-01    C4:   1.5961E-01 C5:     -1.6027E-03                                     C6:   -2.6230E-02    C7:   -1.5437E-01                     C8:     -2.1530E-02                                     C9:   6.7035E-01    C10:  -5.9254E-01    ______________________________________

The substrate of the first diffractive optical element 16 is BK7 Shottglass which is 0.5 mm thick. The c_(x) polynomial coefficients for thediffractive phase profile of the output surface 34 of the firstdiffractive optical element are

    ______________________________________    C1:   -1.9645E-02                     C2:     -1.3703E-01                                     C3:   -1.7417E-02    C4:   6.2476E-02 C5:     1.0119E-02                                     C6:   -8.7547E-03    C7:   -3.8339E-01                     C8:     2.7165E-03                                     C9:   1.8690E-01    C10:  1.3302E+00    ______________________________________

The output surface 34 of the first diffractive optical element 16 isseparated from the stop 19 by a distance 36 of 0.405411 mm. The stop 19is separated from the incident surface 40 of the second diffractiveoptical element 20 by a distance 38 of 0.711957 mm. The c_(x) polynomialcoefficients for the incident or first side 40 of the second diffractiveoptical element are

    ______________________________________    C1:   1.9645E-02 C2:     1.3703E-01                                     C3:   1.7417E-02    C4:   -6.2476E-02                     C5:     -1.0119E-02                                     C6:   8.7547E-03    C7:   3.8339E-01 C8:     -2.7165E-03                                     C9:   -1.8690E-01    C10:  -1.3302E+00    ______________________________________

The substrate of the second diffractive optical element 20 is BK7 Shottglass which is 0.5 mm thick. The c_(x) polynomial coefficients for thediffractive phase profile of the output surface 44 of the seconddiffractive optical element are

    ______________________________________    C1:   1.6929E-01 C2:     -3.5436E-01                                     C3:   2.6430E-01    C4:   -1.5961E-01                     C5:     1.6027E-03                                     C6:   2.6230E-02    C7:   1.5437E-01 C8:     2.1530E-02                                     C9:   -6.7035E-01    C10:  5.9254E-01    ______________________________________

The output surface 44 of the second diffractive optical element 20 isseparated from the spot 24 on the image plane 26 by a distance 46 of1.152816 mm. The image field 50 is 0.5 mm. The magnification ratio is 1to 1.

The spot size 24 is 0.78 microns with a Strehl ratio of 0.9.

The total length distance 47 of the optical focusing system 10 from thelaser source 12 to the spot 24 on the image plane 26 is 4.423 mm.

As noted, the two diffractive optical elements 16 and 20 aresymmetrical. The diffractive phase profile of the incident side 30 ofthe first diffractive optical element 16 corresponds to the diffractivephase profile of the output side 44 of the second diffractive opticalelement 20. The diffractive phase profile of the output side 34 of thefirst diffractive optical element 16 corresponds to the diffractivephase profile of the incident side 40 of the second diffractive opticalelement 20.

EXAMPLE 3:

The wavelength of the emitted light beam 14 from the laser source 12 is780 nm. The object field 48 is 1.0 mm. The distance 28 from the lasersource 12 to the incident surface 30 of the first diffractive opticalelement 16 is 2.97549 mm. The c_(x) polynomial coefficients for thediffractive phase profile of the incident surface 30 of the firstdiffractive optical element are

    ______________________________________    C1:   -8.4398E-02                     C2:     4.4611E-02                                     C3:   -8.2465E-03    C4:   1.2456E-03 C5:     -3.6589E-06                                     C6:   -1.3047E-05    C7:   -1.9141E-05                     C8:     -5.7785E-07                                     C9:   5.3188E-06    C10:  -1.1916E-06    ______________________________________

The substrate of the first diffractive optical element 16 is BK7 Shottglass which is 1.0 mm thick. The c_(x) polynomial coefficients for thediffractive phase profile of the output surface 34 of the firstdiffractive optical element are

    ______________________________________    C1:   -9.7413E-03                     C2:     -1.7575E-02                                     C3:   -6.5718E-04    C4:   4.6921E-04 C5:     5.7003E-05                                     C6:   -5.9202E-06    C7:   -5.4501E-05                     C8:     -3.1309E-08                                     C9:   6.7083E-07    C10:  3.1662E-06    ______________________________________

The output surface 34 of the first diffractive optical element 16 isseparated from the stop 19 by a distance 36 of 0.951054 mm. The stop 19is separated from the incident surface 40 of the second diffractiveoptical element 20 by a distance 38 of 1.336717 mm. The c_(x) polynomialcoefficients for the incident or first side 40 of the second diffractiveoptical element are

    ______________________________________    C1:   9.7413E-03 C2:     1.7575E-02                                     C3:   6.5718E-04    C4:   -4.6921E-04                     C5:     -5.7003E-05                                     C6:   5.9202E-06    C7:   5.4501E-05 C8:     3.1309E-08                                     C9:   -6.7083E-07    C10:  -3.1662E-06    ______________________________________

The substrate of the second diffractive optical element 20 is BK7 Shottglass which is 1.0 mm thick. The c_(x) polynomial coefficients for thediffractive phase profile of the output surface 44 of the seconddiffractive optical element are

    ______________________________________    C1:   8.4398E-02 C2:     -4.4611E-02                                     C3:   8.2465E-03    C4:   -1.2456E-03                     C5:     3.6589E-06                                     C6:   1.3047E-05    C7:   1.9141E-05 C8:     5.7785E-07                                     C9:   -5.3188E-06    C10:  1.1916E-06    ______________________________________

The output surface 44 of the second diffractive optical element 20 isseparated from the spot 24 on the image plane 26 by a distance 46 of2.297549 mm. The image field 50 is 1.0 mm. The magnification ratio is 1to 1.

The spot size 24 is 0.78 microns with a Strehl ratio of 0.9.

The total length distance 47 of the optical focusing system 10 from thelaser source 12 to the spot 24 on the image plane 26 is 8.882869 mm.

As noted, the two diffractive optical elements 16 and 20 aresymmetrical. The diffractive phase profile of the incident side 30 ofthe first diffractive optical element 16 corresponds to the diffractivephase profile of the output side 44 of the second diffractive opticalelement 20. The diffractive phase profile of the output side 34 of thefirst diffractive optical element 16 corresponds to the diffractivephase profile of the incident side 40 of the second diffractive opticalelement 20.

As shown by these illustrative examples, the optical focusing system ofthe present invention using two symmetrical diffractive optical elementsforms a spot with a size under 1 micron and has a Strehl ratio of 0.9.The optical focusing system is flat-field, compact, and consists of onlya few inexpensive, easy to manufacture and easy to assemble opticalelements.

While the invention has been described in conjunction with specificembodiments, it is evident to those skilled in the art that manyalternatives, modifications and variations will be apparent in light ofthe foregoing description. Accordingly, the invention is intended toembrace all such alternatives, modifications and variations as fallwithin the spirit and scope of the appended claims.

I claim:
 1. An optical focusing system comprisinga laser light sourcefor emitting a light beam of a single wavelength, a first diffractiveoptical element for diffracting said light beam, said first diffractiveoptical element being circularly symmetric, a second diffractive opticalelement, said second diffractive optical element being circularlysymmetric and symmetrical to said first diffractive optical element, fordiffracting said light beam from said first diffractive optical elementto a spot, wherein the object field of said optical focusing system is0.5 millimeters and the image field of said optical focusing system is0.5 millimeters and wherein said spot size is less than one micron andsaid optical focusing system has a Strehl ratio of over 0.9.
 2. Theoptical focusing system of claim 1 wherein the first diffractive opticalelement has a first surface with a diffractive phase profile and asecond surface with a diffractive phase profile and wherein the seconddiffractive optical element has a first surface with a diffractive phaseprofile and a second surface with a diffractive phase profile, furtherwherein each of the diffractive phase profiles, Φ, with regard to aposition, ρ, on the surface is defined by

    Φ(ρ)=c.sub.1 ρ.sup.2 +c.sub.2 ρ.sup.4 +c.sub.3 ρ.sup.6 +c.sub.4 ρ.sup.8 +c.sub.5 ρ.sup.10 +c.sub.6 ρ.sup.12 +c.sub.7 ρ.sup.14 +c.sub.8 ρ.sup.16 +c.sub.9 ρ.sup.18 +c.sub.10 ρ.sup.20

where c_(x) are the polynomial coefficients.
 3. The optical focusingsystem of claim 2 wherein the polynomial coefficients of the diffractivephase profile of the first surface of the first diffractive opticalelement are

    ______________________________________    C1:   -1.6880E-01                     C2:     3.5689E-01                                     C3:   -2.6389E-01    C4:   1.5943E-01 C5:     -1.8734E-03                                     C6:   -2.6720E-02    C7:   -1.5680E-01                     C8:     -1.8935E-02                                     C9:   6.9715E-01    C10:  -6.2472E-01,    ______________________________________

further wherein the polynomial coefficients of the diffractive phaseprofile of the second surface of the first diffractive optical elementare

    ______________________________________    C1:   -1.9483E-02                     C2:     -1.4060E-01                                     C3:   -2.1030E-02    C4:   6.0059E-02 C5:     2.9186E-02                                     C6:   -1.2125E-02    C7:   -4.4647E-01                     C8:     -1.0259E-03                                     C9:   8.7927E-02    C10:  1.6600E+00,    ______________________________________

further wherein the polynomial coefficients of the diffractive phaseprofile of the first surface of the second diffractive optical elementare

    ______________________________________    C1:   1.9483E-02 C2:     1.4060E-01                                     C3:   2.1030E-02    C4:   -6.0059E-02                     C5:     -2.9186E-02                                     C6:   1.2125E-02    C7:   4.4647E-01 C8:     1.0259E-03                                     C9:   -8.7927E-02    C10:  -1.6600E+00,    ______________________________________

and further wherein the polynomial coefficients of the diffractive phaseprofile of the second surface of the second diffractive optical elementare

    ______________________________________    C1:   1.6880E-01 C2:     -3.5689E-01                                     C3:   2.6389E-01    C4:   -1.5943E-01                     C5:     1.8734E-03                                     C6:   2.6720E-02    C7:   1.5680E-01 C8:     1.8935E-02                                     C9:   -6.9715E-01    C10:  6.2472E-01.    ______________________________________


4. The optical focusing system of claim 2 wherein the polynomialcoefficients of the diffractive phase profile of the first surface ofthe first diffractive optical element are

    ______________________________________    C1:   -1.6929E-01                     C2:     3.5436E-01                                     C3:   -2.6430E-01    C4:   1.5961E-01 C5:     -1.6027E-03                                     C6:   -2.6230E-02    C7:   -1.5437E-01                     C8:     -2.1530E-02                                     C9:   6.7035E-01    C10:  -5.9254E-01,    ______________________________________

further wherein the polynomial coefficients of the diffractive phaseprofile of the second surface of the first diffractive optical elementare

    ______________________________________    C1:   -1.9645E-02                     C2:     -1.3703E-01                                     C3:   -1.7417E-02    C4:   6.2476E-02 C5:     1.0119E-02                                     C6:   -8.7547E-03    C7:   -3.8339E-01                     C8:     2.7165E-03                                     C9:   1.8690E-01    C10:  1.3302E+00,    ______________________________________

further wherein the polynomial coefficients of the diffractive phaseprofile of the first surface of the second diffractive optical elementare

    ______________________________________    C1:   1.9645E-02 C2:     1.3703E-01                                     C3:   1.7417E-02    C4:   -6.2476E-02                     C5:     -1.0119E-02                                     C6:   8.7547E-03    C7:   3.8339E-01 C8:     -2.7165E-03                                     C9:   -1.8690E-01    C10:  -1.3302E+00,    ______________________________________

and further wherein the polynomial coefficients of the diffractive phaseprofile of the second surface of the second diffractive optical elementare

    ______________________________________    C1:   1.6929E-01 C2:     -3.5436E-01                                     C3:   2.6430E-01    C4:   -1.5961E-01                     C5:     1.6027E-03                                     C6:   2.6230E-02    C7:   1.5437E-01 C8:     2.1530E-02                                     C9:   -6.7035E-01    C10:  5.9254E-01.    ______________________________________


5. An optical focusing system comprisinga laser light source foremitting a light beam of a single wavelength, a first diffractiveoptical element for diffracting said light beam, said first diffractiveoptical element being circularly symmetric, a second diffractive opticalelement, said second diffractive optical element being circularlysymmetric and symmetrical to said first diffractive optical element, fordiffracting said light beam from said first diffractive optical elementto a spot, wherein the object field of said optical focusing system is1.0 millimeters and the image field of said optical focusing system is1.0 millimeters and wherein said spot size is less than one micron andsaid optical focusing system has a Strehl ratio of over 0.9.
 6. Theoptical focusing system of claim 5 wherein the first diffractive opticalelement has a first surface with a diffractive phase profile and asecond surface with a diffractive phase profile and wherein the seconddiffractive optical element has a first surface with a diffractive phaseprofile and a second surface with a diffractive phase profile, furtherwherein each of the diffractive phase profiles, Φ, with regard to aposition, ρ, on the surface is defined by

    Φ(ρ)=c.sub.1 ρ.sup.2 +c.sub.2 ρ.sup.4 +c.sub.3 ρ.sup.6 +c.sub.4 ρ.sup.8 +c.sub.5 ρ.sup.10 +c.sub.6 ρ.sup.12 +c.sub.7 ρ.sup.14 +c.sub.8 ρ.sup.16 +c.sub.9 ρ.sup.18 +c.sub.10 ρ.sup.20

where c_(x) are the polynomial coefficients.
 7. The optical focusingsystem of claim 6 wherein the polynomial coefficients of the diffractivephase profile of the first surface of the first diffractive opticalelement are

    ______________________________________    C1:   -8.4398E-02                     C2:     4.4611E-02                                     C3:   -8.2465E-03    C4:   1.2456E-03 C5:     -3.6589E-06                                     C6:   -1.3047E-05    C7:   -1.9141E-05                     C8:     -5.7785E-07                                     C9:   5.3188E-06    C10:  -1.1916E-06,    ______________________________________

further wherein the polynomial coefficients of the diffractive phaseprofile of the second surface of the first diffractive optical elementare

    ______________________________________    C1:   -9.7413E-03                     C2:     -1.7575E-02                                     C3:   -6.5718E-04    C4:   -4.6921E-04                     C5:     5.7003E-05                                     C6:   -5.9202E-06    C7:   -5.4501E-05                     C8:     -3.1309E-08                                     C9:   6.7083E-07    C10:  3.1662E-06,    ______________________________________

further wherein the polynomial coefficients of the diffractive phaseprofile of the first surface of the second diffractive optical elementare

    ______________________________________    C1:   9.7413E-03 C2:     1.7575E-02                                     C3:   6.5718E-04    C4:   -4.6921E-04                     C5:     -5.7003E-05                                     C6:   5.9202E-06    C7:   5.4501E-05 C8:     3.1309E-08                                     C9:   -6.7083E-07    C10:  -3.1662E-06,    ______________________________________

and further wherein the polynomial coefficients of the diffractive phaseprofile of the second surface of the second diffractive optical elementare

    ______________________________________    C1:   8.4398E-02 C2:     -4.4611E-02                                     C3:   8.2465E-03    C4:   -1.2456E-03                     C5:     3.6589E-06                                     C6:   1.3047E-05    C7:   1.9141E-05 C8:     5.7785E-07                                     C9:   -5.3188E-06    C10:  1.1916E-06.    ______________________________________